This invention relates to a semi-conducting pyrolyzed glass fiber layer covering an insulated electrical conductor which prohibits the development of a corona discharge when an electrical potential exists between the conductor and region adjacent the exterior surface of the insulator.
In many electrical devices, an electrical potential exists between a conductor and the regions immediately adjacent the exterior surface of an insulator surrounding the conductor. In a high powered electrical apparatus such as a dynamoelectric machine, the stationary armature core is generally made of laminations which define a cylindrical bore and which also define circumferentially spaced radial slots opening into the bore and axially extending substantially the length of the stator core. Heavily insulated electrical windings, or armature bars, are disposed in the slots. A high electrical potential difference exists between the windings or armature bars and the members of this stator defining the slots which are at an electrical ground.
The aforementioned large electrical potential difference may be sufficient to produce ionization of the gaseous medium in the region adjacent the exterior surface of the insulation surrounding the armature bars. This ionization tends to initiate arcing along the surface of the insulated armature bars which bridges the insulated path from the windings to the grounded stator laminations.
A similar problem has been recognized in the end turn region of the dynamoelectric machines. In that region, the insulated armature bars extend beyond the respective slots and one set of bars is circumferentially bent from top to bottom, to circumferentially displaced slot positions so as to provide a connection between one bar and another circumferentially spaced bar in the stator.
The ionization of the gas immediately adjacent the insulation of the armature bars is recognized as the production of a corona discharge. In the past, corona has been avoided by wrapping the insulated armature bars with a grounding tape which bleeds off the electric charge developed on the exterior surface of the insulator. The grounding tape is in electrical contact with the stator laminations. The grounding tape longitudinally extends the axial length of the bar in the stator slot as well as extends into a portion of the end turn region beyond the slot.
It is common to place the taped insulated armature bars in a resin bath and vacuum pressure impregnate the resin into the entire structure. This procedure may result in a change of the value of resistivity per square of the grounding tape, therefore, the ability of the tape to prevent corona is not entirely preserved. The change in resistivity of the grounding tape, when acted upon by resin, is well known in the art. Also, the grounding tape may abrade due to the vibration of the armature bar in the stator slot.
The development of a corona discharge in the region adjacent an insulated winding is not limited to dynamoelectric machines since this phenomenon has been noted in other electromagnetic machines such as large AC motors. In those machines, the insulated windings are disposed in longitudinally extending slots in the stator. Commonly, grounding tape is placed on the exterior surface of the insulation and is electrically in contact with the members of the stator defining the slot to minimize the possibility of a corona discharge similar in nature to that which occurs in dynamoelectric machines.
The development of a corona discharge and the resulting possibility of a flash over between the windings and the electrical ground adjacent to the exterior surface of the insulator is one extreme of the general problem of electrical charge buildup on the exterior of an insulated winding. This buildup of electrical charge is sometimes caused by the capacitive characteristics of the device. For example, cables carrying high voltages are sometimes subject to charge build-up on the exterior surface of the cable's insulation. Also, insulated conductors in transformers are affected b this capacitive charge build-up. The charge may also be a static electric charge. The grounding tapes utilized in prior art devices are not well suited to bleed off this electric charge unless the electrical apparatus is shaped such that the tape is in close contact with the entire insulated surface. For complex shapes, those shapes other than round or rectangular shapes with rounded corners, the grounding tape is not easily placed in close contact. In those situations, the portions of the exterior surface which are not in intimate contact may have an electrical charge buildup thereon since the tape does not bleed off the charge to ground.
Static charge build-up problems have been recognized as affecting sensitive electronic equipment, such as integrated circuit chips utilized in digital electronic equipment. The manufacture of these IC chips is generally sensitive to static charges on the conveyors, handling trays, racks and other means of packaging, handling, mounting and transporting. In the past, conductive paints were utilized to render the part, such as a plastic handling tray, semi-conducting. The tray was electrically grounded by an appropriate means to bleed off the static electric charge.